CFD simulation via conventional Two-Fluid Model of a circulating fluidized bed riser: Influence of models and model parameters on hydrodynamic behavior

Abstract

A two-dimensional (2D) numerical simulation of gas–solids flow in a circulating fluidized bed riser was performed based on the Two-Fluid Model (TFM). In this model, the kinetic theory of granular flows (KTGF) was used to solve gas–solids flow behavior. Despite several published studies, the selection of proper wall boundary conditions (BC) and the appropriate closure model to simulate riser flow remains inconclusive. In this study, a number of simulations were performed to understand the influence of the wall boundary condition, solid shear viscosity model and particle–particle restitution coefficient. At first, we systematically evaluated the influence of the combined effect of the specularity and particle–wall restitution coefficients based on Johnson and Jackson [1] BC. The key point we observed was that the effect of high specularity coefficient (φ=0.001) was less at low particle–wall restitution coefficients (ew=0.8, 0.9) on the axial solid holdup profile. At lower specularity coefficient (φ=0.00001 & 0.0001) value, different particle–wall restitution coefficients (ew=0.8, 0.9, 0.99) did not show any significant effect. Moreover, the effect of the elasticity of particle collisions showed that the low particle–particle restitution coefficients gave good quantitative agreement. The effects of solid shear viscosity models on flow predictions were investigated under different solid circulation rates (Gs=39.14kg/m2s, 51.05kg/m2s, 73.21kg/m2s). Both Syamlal et al. [2] and Gidaspow [3] models gave good qualitative agreement with the experimental results. The Syamlal et al. solid viscosity model demonstrated fairly consistent results in the dilute region, whereas transition from lower to higher solid holdup was reasonably well predicted by the Gidaspow model.